Adding CSS additional class to a block

Each block that you use in the Block editor can have additional CSS classes defined using the block properties. 

1. Navigate to the Block editor by opening an existing page or post, or creating a new page or post as needed.
2. Click on the block that you wish to modify.
3. In the sidebar at right click on the label Block. If you don’t see the sidebar, click on the gear icon in order to make it appear.
4. Scroll down to the Advanced section and then click on the drop-down arrow. 
5. Add your classes to the field labeled Additional CSS class(es).
6. Click on Update or Publish once you are ready.